The present invention relates generally to resist materials employed in the manufacture of integrated circuits and microcomponents. More particularly, the present invention relates to a high resolution, multi-layer resist for use in microlithography for the formation of microelectronic circuits.
The rapid movement toward higher levels of integration in monolithic circuits has been made possible by increased component packing densities and smaller geometries within the devices and circuits. These advances are due principally as a result of advances in lithographic techniques and in resist technology, however limitations in the resists have become the resolution limiting factor.
The minimum feature size is determined by the combination of the capabilities of the system optics and the contrast value of the photoresist. The photoresist contrast values for conventional optical resists are actually quite poor. Theoretical resolution of a typical lithography system must be two or more times greater than the feature size being defined. Thus, the equipment is theoretically capable of resolving features which are finer than those currently achieved. This characteristic suggests that current systems may be used to achieve higher resolution if higher contrast resists were available.
The difficulties in achieving higher resolution may be met by providing thinner resist coatings. However, thinner coatings typically cannot withstand subsequent processing. Thin organic films produced by spin casting are prone to pin holes, which are remedied b compromising thickness. Conventional organic resists are developed with wet developers which often causes resist swelling which, in turn, causes resolution loss, bridging of adjacent features and snaking of narrow lines. Moreover, with wet development schemes, desired materials will not readily dissolve into small spaces due to surface tension exclusion effects.
The problems of the current lithographic techniques are not met solely with higher resolution capabilities. Higher numerical aperture (NA) lenses may provide better resolution, but also result in decreased depth of focus and decreased field size. Reduced depth of focus is a serious deficiency when the systems are employed in modern integrated circuits having multiple levels of metalization. These types of circuits tend to have relatively large changes in surface elevation which may cause some regions to be out of focus. Additionally, conventional spin-on techniques tend to deposit a layer having varying thickness which is thinner at the edges of raised features. These thinner resist areas are then prone to overexposure and a change in line width occurs at the steps between the raised feature and its adjacent depression. The current tri-level resist schemes help to solve these problems but are non-ideal due to the complexity of the deposition and development processes. Tri-layer schemes also employ conventional organic photoresists and will, therefore, suffer from many of the aforementioned problems related to these materials.
With conventional photoresists, reflections from the substrate may create standing waves which reduce photo speed and create an uneven exposure in the resist. If the underlying layer is highly reflective, incident light is reflected laterally into the resist where the layer passes over steps. This leads to a change of line width at the steps known as "reflective notching".
Thus, the resist systems currently employed in microlithographic techniques suffer from a variety of problems which may be met by providing a high contrast resist system which can be dry developed, a resist material which is self-planarizing to avoid focus problems and which facilitates deposition of a uniformly thick photoactive layer, and which has a light absorbing photoactive region at the surface to guard against reflective notching. Additionally, the deficiencies of the current multi-layer resists dictate their solution by a new multi-layer resist scheme having enhanced high resolution capability and which will exhibit utility using a wide range of exposure methods including without limitation, light, x-ray, laser, ion beam and electron beam systems.